A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive traces, pads and other features etched from electrically conductive sheets, such as copper sheets, laminated onto a non-conductive substrate. Multi-layered printed circuit boards are formed by stacking and laminating multiple such etched conductive sheet/non-conductive substrate. Conductors on different layers are interconnected with plated-through holes called vias.
A printed circuit board includes a plurality of stacked layers, the layers made of alternating non-conductive layers and conductive layers. The non-conductive layers can be made of prepreg or base material that is part of a core structure, or simply core. Prepreg is a fibrous reinforcement material impregnated or coated with a thermosetting resin binder, and consolidated and cured to an intermediate stage semi-solid product. Prepreg is used as an adhesive layer to bond discrete layers of multilayer PCB construction, where a multilayer PCB consists of alternative layers of conductors and base materials bonded together, including at least one internal conductive layer. A base material is an organic or inorganic material used to support a pattern of conductor material. A core is a metal clad base material where the base material has integral metal conductor material on one or both sides. A laminated stack is formed by stacking multiple core structures with intervening prepreg and then laminating the stack. A via is then formed by drilling a hole through the laminated stack and plating the wall of the hole with electrically conductive material, such as copper. The resulting plating interconnects the conductive layers in the laminated stack.
In some applications, it is desirable to form part of the printed circuit board with a reduced number of layers, which are flexible, to form a flexible portion that is bendable yet remains interconnected to other rigid portions of the printed circuit board, thereby forming a rigid-flexible printed circuit board. Current process flow is to pre-cut prepreg at a desired flexible portion and then control resin squeeze out during the lamination process. This process flow has disadvantages such as high cost of low flow prepreg, limited supply of low flow prepreg and difficulty in controlling resin squeeze out. Additionally, lamination accessories such as release film and conformal film are needed which also add cost. Release film provides a separation between a surface copper layer (conducting layer) in the lamination stack and the conformal film. Conformal film is a thermoplastic layer which softens under lamination temperature and conforms to the area with prepreg pre-cut. This reduces prepreg resin flowing into the flexible portion. However, resin can still flow into the rigid-flexible boundary randomly, resulting in an irregular rigid-flexible boundary. Such an irregular boundary forms a serrated surface that cuts against the flexible portion. Further, lamination under high pressure and the impact of conformal film can result in increased panel distortion and it is difficult to achieve flat surface for fine line etching or even dielectric thickness across panel to control impedance. A panel here refers to the finished product of the stack of laminate and prepreg after lamination. In order to solve these issues, a new manufacturing process for rigid-flex printed circuit boards is needed.